1. Field of the Invention
This invention relates to an illumination device in which a discharge tube is caused to emit a light by a high frequency electromagnetic field being applied to the discharge tube from the outside thereof.
The invention further relates to an original reading apparatus for reading the image of an original by moving the illumination device in which the discharge tube is caused to emit a light by extraneous application.
2. Related Background Art
Fluorescent lamps and halogen lamps have heretofore been widely used in original reading apparatuses and usual illumination.
A fluorescent lamp produces visible light and, wavelength of the it can emit a selected wavelength by selection of a fluorescent material and is the preferred illumination source. But if a great current is applied to the filament to obtain a great quantity of light, the filament will be immediately burnt out and therefore, the quantity of light obtained is low. Also, if a current flows to the filament, deterioration will progress due to the excited gas in the discharge tube and thus, the service life of the filament itself is short.
As compared with a fluorescent lamp, a halogen lamp can provide a great quantity of light, but as shown in FIG. 24 of the accompanying drawings, it produces a great deal of light in a wavelength range other than visible light. That is, a halogen lamp produces a great deal of light not used in an original reading apparatus or an apparatus utilizing chiefly the wavelength range of about 400-800 nm such as a copying apparatus having a photosensitive medium, and therefore is low in power efficiency. Also it produces light by converting electrical energy into heat, and this leads to a great deal of heat generation.
In view of such problems, a light source has been developed which utilizes the discharge phenomenon like a fluorescent lamp and which has a much higher brightness and a much longer service life than a fluorescent lamp. Such a light source applies energy to a discharge tube from the outside thereof, as and is known from Japanese Laid-Open Patent Application Nos. 98457/1980 and 249,240/1985.
FIG. 23 of the accompanying drawings is a cross-sectional view of an example of such a light source. In FIG. 23, reference numeral 4 designates a lamp to the inner surface of which a fluorescent material 3 is applied and in which mercury and an inert gas are enclosed. The lamp 4 is formed with a cylindrical portion 7 protruding so as to include a transformer 2. The transformer 2 comprises a core 6 and a coil 5, and the ends of the coil 5 wound around the core 6 are connected to a high frequency lamp source 1.
By a high frequency voltage being applied from the high frequency lamp source 1 to the coil 5, a high frequency electromagnetic field is produced around the coil 5. The electrical energy of this electromagnetic field excites the mercurly gas in the lamp 4, and the ultraviolet rays of the mercury produced by this excitation are changed into a visible light by the fluorescent material 3 applied to the inner surface of the lamp 4.
Such a light source utilizes the discharge phenomenon and can provide a light of an appropriate wavelength range by the selection of the fluorescent material and does not have a filament emitting heat electrons, but utilizes electromagnetic field energy applied by an electrode provided in contact with the outer wall of the discharge tube and therefore permits the application of a great amount electric power, and is of high brightness and enjoys a long service life because the electrode is not exposed to the excited gas in the discharge tube.
However, while such a light source has the merits of high brightness, long service life and good power efficiency, it suffers the adverse effects of high frequency noise because of its utilizing a high frequency wave. Thus, where such an illumination device utilizing a high frequency electromagnetic field is used in an apparatus having electronic parts such as an original reading apparatus, there has arisen a problem that the device malfunctions or does not operate.